a) Field of the Invention
This invention relates to curable thiol-ene compositions based on bicyclic 'ene compounds having improved stability as compared to such compositions known heretofore.
b) Description of the Related Art
The synthesis of polythioethers by crosslinking polyenes with polythiols is a process which is well recognised in the art. The curing process is frequently one of photopolymerization initiated by actinic light, in particular light in the UV-visible range. Polymerization may also be effected by the use of high energy radiation and by exposure of the reactants to chemical free radical generating agents, usually thermally activated.
A particularly preferred group of thiol-ene compositions are those in which the 'ene is a bicyclic compound such as norbornene. Such compositions are described in U.S. Pat. No. 4, 808,638, the contents of which are incorporated herein by reference.
Thiol-norbornene compositions cure to polythioethers which have advantageous physical properties. However the thiol-norbornene compositions have poor stability characteristics in respect of both formulations designed to be photopolymerized and formulations designed to be thermally polymerized. In particular, the formulations have a short shelf-life and are prone to "set-up" even when stored in the dark and even when there is no photoinitiator in the formulation. Such premature curing therefore cannot be attributed to "stray light". The polymerization mechanism of polyene-polythiol systems is not fully understood. Consequently, the reason for formulation instability in the dark at ambient temperature is also unclear.
Some thiol-ene compositions are available which claim to have a shelf life of 4-6 months at room temperature. However a shelf life of 1 year at room temperature is desirable. In an effort to prolong the shelf-life of the known formulations it is recommended that the formulations be stored refrigerated. However, refrigeration of large quantities of polythiol-polyene formulations is costly and space-consuming.
Conventional stabilising additives have been included in norbornene-thiol (nene-thiol) compositions with little success. Table I below lists numerous materials which have been examined as stabilizers in organic nene-thiol systems and have been found to have little or no effect on the stability of the formulations.
TABLE I ______________________________________ MATERIALS HAVING LITTLE IF ANY BENIFICAL EFFECT ON THE STABILITY OF ORGANIC NENE-THIOL SYSTEMS ______________________________________ Phenol Based Phenothiazine Hydroquinone 3,6-Di-tert-butyl-4-methyl phenol (BHT) 4,4'-Methylene bis(2,4-Di-tert-butyl phenol) Irganox 1010 (Ciba Geigy) Irganox 1076 (Ciba Geigy) Irganox MD-1024 (Ciba Geigy) Vitamin E 1,4-Naphthoquinone Pyragallol p-Methoxy phenol pH Modifiers Acrylic Acid Citraconic acid Acetic acid 1-Hydroxyethylidene-1-diphosphonic acid (Dequest 2010) Phosphorous acid 2-(Di-isopropylamino) ethanol Sacrificial Mercaptans Thiophenol p-Hydroxy thiophenol 4-Bromo thiophenol 4-Methoxy benzenethiol 2-Mercapto benzothiazole m-Trifluoromethylthiophenol 2-Mercapto benzoxazole 2-Nitrobenzyl mercaptan Durene dithiol Amine Based Stabilisers 2,4,6 Tris (dimethylaminomethyl) phenol 3-Dimethylaminophenol N,N'-di-sec-butyl phenylene diamine N-phenyl naphthylamine Acetyl phenyl hydrazine Tinuvin 770 (Ciba Geigy) Tinuvin 1130 (Ciba Geigy) Sacrificial Ene Monomers N-Vinylpyrrolidone n-butyl vinyl ether N-Vinylcarbazole N-Vinylcaprolactam Florstab U.V.3 (Floridienne U.K. Ltd.) Metal Chelators Ethylene diamine tetraacetic acid, tetrasodium salt N-(2-Hydroxyethyl) Ethylene diamine triacetic acid, trisodium salt Phosphorous or Boron Based Stabilizers Tributyl phosphine Diphenyl phosphine Triphenyl phosphine Tribenzyl phosphine Allyl diphenyl phosphine Triallyl phosphine Triethyl phosphite Weston 600 (Spiro phosphite) Hydroxy methyl bicyclic phosphite Trinorborane boron Triphenyl boron ______________________________________
The organic nene-thiol compositions referred to above contain norbornenes and thiols having a structure based on carbon atoms optionally interrupted by one or more hetero atoms such as 0, S or N. It is also possible to make useful compositions in which both the norbornene and the thiol are based on siloxane backbones. Such compositions (hereafter called "inorganic nene-thiol compositions") have different stability characteristics from the organic nene-thiol compositions and may, for example, be stabilised satisfactorily by 3,6-di-tert.-butyl-4-methyl phenol (BHT).
The present invention is based upon the unpredictable discovery that non-acidic nitroso compounds are successful as stabilisers in both organic and inorganic norbornene-thiol compositions.
The use of nitroso compounds has also been described to inhibit polymerization in non-thiol containing systems. According to the Description of prior art in U.S. Pat. No. 4,168,982, Hungarian Patent Specification No. 150,550 (1963) describes the use of p-aminonitroso-benzene and alpha-nitroso-beta-naphthol as free radical polymerization inhibitors in styrene systems; U.S. Patent Specification No. 3,625,696 describes thermal polymerization inhibition by N-nitrosocyclohexylhydroxylamine salts; inhibition of styrene and vinyl acetate polymerizations by the use of 4-nitrosophenol, 1,4-dinitrosobenzene, nitrosoresorcinol, p-nitrosodimethylaniline and other nitroso compounds are described in Chimia (Aaray), 19, p.116 (1965), and Kinetics and Catalysis (USSR), 6, p.175-181 (1965); and U.S. Pat. No. 3,203,801 describes the use of N-substituted p-nitrosoanilines as sensitometric modifiers for photopolymerization systems.
Similarly U.S. Pat. No. 4,168,982 itself describes the use of nitroso dimers to selectively inhibit thermally-induced free radical polymerization. The nitroso dimers dissociate to form the active nitroso monomer which produces the stabilizing effect. In the photographic and printing industry, stabilizers which inhibit dark reactions (i.e. thermal polymerization) without inhibiting photopolymerization are known. Japanese Patent Application Nos. 88/170401 (Wade et al), 86/103150 (Fujikawa et al), 86/57271 (Ishii et al), 86/481 (Kuroda), 86/4578 (Kuroda), 79/31727 (Hosaka et al) and 74/59892 (Manamii et al), describe the use of N-nitrosophenylhydroxylamine derivatives as polymerisation inhibitors. In particular, the aluminium salt of N-nitrosophenylhydroxylamine has been described for use as a stabiliser in vinyl polymer compositions (88/170401), the production of photosensitive printing plates comprising polyvinylacetate and acrylates (86/103150), the production of embossed decorative materials using inks comprising PVC and trimethylolpropane trimethacrylate (86/57271), embossed sheets again using PVC sheets with a photosensitive resin coating (86/841, 86/4578), the production of photosensitive resin (79/31727) and in unsaturated polyester compositions (74/59892).
Japanese Patent Application No. 79/106307 describes nitrosophenylhydroxylamine salts, in particular, the aluminium salt of N-nitrosophenylhydroxylamine as polymerization inhibitors for photocurable printing inks comprising Epikote 828 and acrylic acid.
European Patent Application 0,289,852 A1 describes a radiation curable pressure sensitive adhesive composition comprising:
(A) 100 parts by weight of a hydrogenated polybutadiene applied oligomer which has one or more ethylenically unsaturated terminal radicals in its molecule and in which 70% or more of intramolecular carbon-carbon double bonds has been hydrogenated; PA1 (B) from 0.2 to 20 parts by weight of a chain transfer agent; and PA1 (C) from 0.001 to 1.0 parts by weight of a thermal polymerization inhibitor selected from metal complexes of N-nitrosophenylhydroxylamine. Such a composition is said to have excellent storage stability and curing property. Among illustrative examples of chain transfer agents are mentioned thiols such as pentaerythritol tetrakis (.beta.-thiopropionate). The preferred amount of the chain transfer agent is from 2 to 15 parts by weight per 100 parts by weight of the oligomer. If this amount exceeds 20 parts by weight, the curing is stated to be insufficient. If the amount of the thermal polymerization inhibitor exceeds 1.0 parts by weight, the radiation curing is stated to be inhibited. The preferred range is from 0.01 to 0.1 parts by weight. PA1 i) a 'ene compound having a plurality of groups of the formula I: ##STR1## where Q is CR.sup.1.sub.2, O , S, NR.sup.1 or SO.sub.2, R.sup.1 (which may be the same or different) is H or alkyl, and m is 0-10; PA1 ii) a compound having at least two pendant or terminally positioned --SH functional groups per average molecule; and PA1 iii) a stabilizing amount of a non-acidic Nitroso compound.
European Patent Application 0,289,852 A1 is concerned with a pressure sensitive adhesive composition of a particular type, having a polybutadiene backbone. A relatively small proportion of a thiol is suggested as a chain transfer agent. However there is no teaching concerning thiol-ene compositions based on bicyclic 'ene compounds such as norbornene which are particularly unstable on storage or about compositions in which 'ene and thiol groups are in molar equivalence. In particular, it could not be predicted that nitroso compounds could be used as stabilisers in such compositions without adversely affecting curing performance.
In view of the wide range of stabilizers previously examined for use with norbornene-thiol compositions it could not be predicted that N-nitrophenylhydroxylamines would be of any utility.